Home computers to search for tangoing pulsars

The idle time of hundreds of thousands of computers will be put to work to look for pulsars orbiting black holes or other neutron stars. The search, part of a distributed computing program called Einstein@Home, could turn up extreme pairs of astrophysical objects that could put general relativity to the most stringent tests yet.

These ripples in the fabric of space-time are thought to be emitted by dense, moving objects such as spinning neutron stars or colliding black holes according to Einstein's general theory of relativity. But so far no gravitational waves have ever been observed. Einstein@Home searches for their signals in data from the Laser Interferometer Gravitational wave Observatory (LIGO) and the British-German GEO-600 gravitational wave observatory.

Now, the software has been expanded to comb data from the Arecibo Observatory in Puerto Rico to look for star systems that boast at least one pulsar – a spinning neutron star that shines bright beams of light from its poles.

Close-knit pairs

Pulsars tend to orbit any partners they have at close distances, where strong gravitational fields are at work. That makes them ideal laboratories to test a number of predictions of general relativity and potentially reveal deviations from the long-held theory.

So far, just nine pulsars have been found orbiting other neutron stars. The closest such pair, in which the objects orbit each other every 2.4 hours, was used in 2008 to measure how much a star's spin axis wobbles as a result of the gravitational warping of space. The observation, made over a period of four years, agreed with the prediction of general relativity to within 13%.

Finding pulsars in even tighter embraces would allow astrophysicists to test the theory with greater precision. "The more compact the binary the better," says Jim Cordes of Cornell University in Ithaca, New York, a member of a collaboration that has been searching Arecibo data for such pairs.

Best fit

To search for the extreme binaries, Einstein@Home will use a program to examine the lighthouse-like signals of pulsars, whose light beams periodically sweep across the Earth.

If the pulsar is orbiting another star, the wavelengths of its light will be subtly compressed and stretched as it approaches and recedes along our line of sight. By comparing Arecibo's data with a host of simulated neutron star scenarios, computers can find the best fit and determine whether the pulsar has a companion.

So far, Cordes and his colleagues have had no luck finding such pairs with orbital periods shorter than 2.4 hours. But the team's computing power has been limited to discovering pulsars that take 50 minutes or more to orbit their companion.

Increasing the computing time with Einstein@Home would allow more comparisons to be made between the data and the simulations to find the best fit. This could reveal pulsars that dance around their partners with periods of 11 minutes or perhaps less.

Event horizon

The search could also reveal even more exotic pairings, like a pulsar in orbit around a black hole. "That would be a tremendous discovery because we don't know of any of those yet," Cordes told New Scientist. "Then we can use the pulsar to study space-time around the black hole, and if the geometry is right, we might even be able to say something about the event horizon," a boundary beyond which no matter or light can escape from the black hole.

Although Einstein@Home was set up to find evidence of gravitational waves, the pulsars it finds will not be detectable by existing gravitational wave experiments, says project leader Bruce Allen of the Albert Einstein Institute in Hannover, Germany. Present-day detectors are designed to pick up more dramatic ripples in space-time, such as those released when two neutron stars merge.

Uncertain funding

But Allen says the neutron star binaries found by Einstein@Home could be detectable by the proposed European-US space mission LISA, which is expected to detect the effects of tens of thousands of binary systems. "Any systems we find in this way would probably become a calibration source for LISA," Allen says.

In the meantime, Cordes hopes bringing volunteers into the search will raise visibility for Arecibo, which faces an uncertain future. The US National Science Foundation (NSF) is set to open up a competition to find an organisation to manage the telescope for another five years, beginning in 2010.

But it is unclear how much funding the agency will offer to run the facility. "I think the wild card in all this is how much money NSF is going to offer," Cordes says. "The fear is that they wouldn't put in all that's needed, so that whoever does manage it would have to come up with [the rest]," a prospect that could hamstring the telescope in uncertain economic times.

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The Einstein@Home program uses idle time on computers to search for pulsars in tight orbits around other neutron stars or black holes. These pairs of objects are ideal laboratories for testing the predictions of general relativity (Illustration: Daniel Cantin/DarwinDimensions/McGill University)